Antigens for vaccination and/or diagnostic purposes are typically single antigens from a pathogen, or complex mixtures of multiple known antigens from a pathogen, or of multiple known and unknown antigens of a pathogen such as live, attenuated or inactivated bacteria or viruses. Depending on the particular type of pathogen, single antigens may provide a quantifiable signal in immunodiagnostic tests (antibody or cellular responses). However, due to variations among individuals in their immune response profiles, single antigen tests are often not sufficient to obtain useful diagnostic information with useful specificity and sensitivity.
For example, multiple tests are often required for accurate diagnosis of tuberculosis (TB). Most typically, a person suspected of being infected with M. tuberculosis is tested using the tuberculin skin test (TST) that often provides variable results, which make their interpretation rarely consistent. Alternative tests are the interferon gamma release assays (IGRAs). These tests are more specific that TST but they still do not provide means of distinguishing persons having active tuberculosis from persons who are infected but are not currently harboring an active disease process. For investigation of active TB, a sputum smear test for acid fast bacilli can be employed to identify M. tuberculosis directly, which tends to provide good specificity. However, the sensitivity varies widely among different laboratories. To obtain a more definite result, active TB may be diagnosed by bacterial culture from, e.g., sputum or other bodily fluids. Unfortunately, such test requires a dedicated microbiology laboratory and several weeks to obtain the results. More recently developed methods, such as real time PCR assays are relatively accurate but require sophisticated equipment and highly trained personnel, and they are very susceptible to cross-sample contamination.
Based on the above drawbacks it is therefore desirable to develop an antibody-based test that would overcome at least some of the difficulties associated with bacterial culture, genetic analysis or other known methods, and considerable effort has been spent defining and identifying immunoreactive proteins in membrane fractions of M. tuberculosis and M. tuberculosis-conditioned culture medium (‘culture filtrate proteins’ or CFPs). Candidate antigens are typically tested for diagnostic utility in ELISAs and Western blots using TB sera and sera from healthy controls. CFPs are more widely studied because of the convenience of working with soluble proteins. Of the >100 M. tuberculosis proteins in culture filtrates (representing about 2.5% of the M. tuberculosis proteome), roughly two dozen are recognized by sera from TB patients, most of which have been previously identified. Yet despite these efforts, there remains no effective serological test with the sensitivity and specificity required to accurately diagnose TB, particularly in the early stage of infection. Moreover, none of the heretofore known antigens is generally applicable to differentiate among stages (e.g., active disease versus non-active), secondary infections, etc., as the signal is either impossible to deconvolute (e.g., compound signal from inactivated pathogen) or only provides a single data point.
Similarly, where known antigens are used in a vaccine, numerous problems are known due to the variability of individual immune response and potential prior exposure. More recently, multivalent vaccine preparations have become available where in a single dose, multiple and distinct antigens, from multiple and distinct serotypes, of a single pathogenic organisms were combined (Prevnar™: Heptavalent vaccine against Streptococcus pneumoniae capsular serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F). While such mixed preparations tend to provide a broader range of protection against different serotypes, various difficulties nevertheless remain. Most significantly, where a single antigen fails to elicit an immune response, coverage to the corresponding serotype is not present. Thus, combination of single defined antigens from several serotypes merely combines benefits and problems associated with the single antigens.
Therefore, while numerous methods of identification and use of antigens are known in the art, all or almost all of them suffer from one or more disadvantages. Consequently, there remains a large, unmet need to provide improved compositions and methods of antigens from M. tuberculosis for diagnosis and therapy of TB.